1. Field of the Invention
The present invention relates to an optical encoder, e.g., a linear encoder for the measurement of a straight range or a straight velocity or a rotary encoder for use in a sensor responsive to the angle and angular velocity of a rotary body or for controlling the rotation of, e.g., a photoconductive drum included in a copier.
2. Description of the Background Art
The prerequisite with a conventional optical encoder is that a scale be mounted with utmost precision. Any error in the mounting angle of a scale (linear encoder) or the eccentricity of a scale (rotary encoder) directly translates into a measurement error. Particularly, in the case of a rotary encoder, an error ascribable to eccentricity is as great as an amount of eccentricity divided by the diameter of a scale. Therefore, when resolution is high, a rigid housing and accurate position adjustment are essential and make a rotary encoder extremely expensive.
Today, a rotary encoder implemented as a module for a cost reduction purpose is available and made up of a rotary scale and a sensing head. The user of this type of rotary encoder is expected to assemble the scale and head by using a jig. However, when such a rotary encoder is mounted to general equipment, the scale need accurate mounting and adjustment that increase cost.
In light of this, Japanese Patent Laid-Open Publication No. 7-140844, for example, discloses a rotary encoder using two sensing heads shifted in position from each other by 180xc2x0. The sum of angles measured by the two sensing heads is halved for thereby obviating the mounting error of an encoder wheel and the irregularity of rotation signals. Although this rotary encoder may accurately determine angular velocity, the two sensing heads increase the cost of the encoder and therefore the cost of equipment to which it is applied.
It is therefore an object of the present invention to provide an optical encoder that is low cost, capable of correcting the mounting error of a scale despite that it does not need accurate adjustment at the time of mounting, and applicable even to equipment for which it has heretofore been considered to be not feasible.
It is another object of the present invention to provide a rotary encoder capable of correcting the eccentricity of an encoder wheel with a single sensing head, allowing the encoder wheel to be mounted without any precise adjustment, and determining a rotation angle at low cost.
An optical encoder of the present invention includes a scale movable in a preselected direction and formed with an array of slits in the direction of movement of the scale. A light source emits a light beam toward the scale. A light-sensitive portion transforms light transmitted through or reflected from the scale to a corresponding electric signal. The slits each have a dimension, as measured in the direction perpendicular to the direction of movement of the scale, limited in accordance with the diameter of a beam spot to be formed on the scale. With this configuration, the encoder is low cost and can correct the mounting error of the scale without resorting to precise adjustment at the time of mounting.